Conventionally, cosmetics for eyelashes containing fibers, natural fibers and/or synthetic fibers, for the purpose of making eyelashes look longer (long lash effect) have been used.
However, it is difficult to obtain a sufficient long lash effect simply by adding fibers to a cosmetic for eyelashes. Also, the curling effect is hard to achieve. Therefore, surface treatments of fibers have been investigated (see Patent Citation 1 for example). Furthermore, investigations have been carried out on blending in fibers having forms that facilitate orienting along eyelashes, utilizing fibers having modified cross-sections (see Patent Citation 2).
For cosmetics for eyelashes that aim at increasing the volume of eyelashes, investigations have been carried out about blending in fibers with waves (see Patent Citation 3, for example).
However, even the cosmetics for eyelashes containing surface-treated fibers and/or wave fibers are applied to eyelashes, each fiber's orientation is random and not in line with others. That is, the fibers attach to eyelashes sideways and cannot achieve natural looking eyelashes.
On the other hand, the use of modified cross-section fibers can achieve natural looking eyelashes. However, a reduction in the curling effect is inevitable.
Various patent applications have been filed about manufacturing methods of modified cross-section fibers (see Patent Citation 4 and Patent Citation 5). Hollow fibers have been developed as hollow threads for swimsuits (see p 156-157 in Non-Patent Citation 1, for example), but have never been used for cosmetics for eyelashes.    Patent Citation 1: Japanese Patent Laid-Open No. 2002-154932 bulletin    Patent Citation 2: Japanese Patent Laid-Open No. 2004-107237 bulletin    Patent Citation 3: Japanese Patent Laid-Open No. 2002-308735 bulletin    Patent Citation 4: Japanese Patent Laid-Open H05-132812 bulletin    Patent Citation 5: Japanese Patent Publication H04-033893    Non Patent Citation 1: Haiteku Sen-i no Sekai (World of Hightech Fibers): written by Motomiya, Tatsuya, published by Nikkan Kogyo Shinbun